September 2016 Issue

Eye Health: The Eyes Have It
By Carrie Dennett, MPH, RDN, CD
Today's Dietitian
Vol. 18 No. 9 P. 22

How RDs Can Help Patients See Clearly With the Power of Produce

We often take our sight for granted—until it starts to fade. More than one-half of all Americans develop cataracts by age 80, while age-related macular degeneration (AMD) is the top cause of severe vision loss and blindness in adults older than age 60. Glaucoma also can compromise our vision as we age. Then there's diabetic retinopathy, the leading cause of vision loss and blindness among younger adults.

One risk factor these eye diseases have in common is the oxidative stress that can accumulate with age.1 Our eyes are constantly exposed to ultraviolet (UV) radiation and pollutants, making them vulnerable to damage by free radicals.1 The retina has a higher concentration of polyunsaturated fatty acids than other body tissues, making them particularly prone to oxidation.1 Accordingly, the nutrients that may benefit eyes the most have antioxidant and anti-inflammatory properties and come from fruits and vegetables.

"The rule of thumb I tell all my patients is what's good for you is good for your eyes," says Saysha Blazier, OD, a private practice optometrist in Jersey City, New Jersey, and a member of the American Optometric Association's Health Promotions Committee.

Carotenoids
The most promising research connecting nutrition and eye health involves the carotenoids, the family of nutrients that provide the yellow, orange, and red colors in many fruits and vegetables. Our bodies use beta-carotene to make vitamin A, which is critical for vision. Two other carotenoids, lutein and zeaxanthin, act as natural antioxidants in the eye, protecting it from damaging UV light.2

Research suggests that people who eat lots of leafy greens and a variety of other fruits and vegetables may have less risk of developing cataracts or AMD. For example, researchers recently examined data from more than 100,000 subjects enrolled in the Nurses' Health Study and the Health Professionals Follow-Up study. The group who had the highest blood levels of lutein and zeaxanthin were about 40% less likely to develop advanced AMD than the group with the lowest levels. Other carotenoids seemed to provide a 25% to 35% risk reduction.3

On the supplement front, the Age-Related Eye Disease Study 2 (AREDS2), conducted by the National Institutes of Health's (NIH) National Eye Institute, examined the effects of a few "cocktails" of vitamins and minerals on the risk of cataracts and advanced AMD.4,5 One cocktail included vitamin C (500 mg), vitamin E (400 IU), lutein (10 mg), zeaxanthin (2 mg), zinc oxide (25 mg), and copper/cupric oxide (2 mg).4

This AREDS2 supplement cocktail didn't prevent cataracts or onset of AMD, but it did reduce risk of progressing to advanced AMD by about 30%.4,6 Those who benefitted most had the least healthful diets and didn't eat many lutein- and zeaxanthin-rich foods. This was also the only group who saw a reduced need for cataract surgery.5 Despite the demonstrated benefits of lutein for AMD and cataracts, the ARIC (Atherosclerosis Risk In Communities) study found no association between estimated lutein intake and incidence of diabetic retinopathy.7

Antioxidants
Although it has been suggested that a diet rich in antioxidant vitamins and minerals (vitamin C, vitamin E, selenium, and zinc) along with carotenoids may reduce the risk of developing AMD, a 2012 Cochrane review—which included four large high-quality randomized controlled trials—concluded that the evidence is insufficient.8 A separate review in the same year found that antioxidants may be of modest benefit for slowing the progression of AMD.9

Another 2012 Cochrane review found a lack of evidence that supplementing with vitamin C, vitamin E, and beta-carotene—alone or in combination—prevents or slows the progression of age-related cataracts. The doses in each trial reviewed were higher than the recommended dietary allowance.10

On the other hand, a recent study of female twins found that women who reported consuming more vitamin C-rich foods had a 33% risk reduction of cataract progression over the decade.11 The fluid inside the eyeball is normally high in a compound similar to vitamin C, so scientists believe that dietary vitamin C may provide extra protection against the oxidation that can cause clouding of the eye's normally clear lens.11

Oxidative stress exacerbates diabetic retinopathy, but several studies have found no protective effect from beta-carotene, vitamin C, or vitamin E.1 A five-year follow-up study in Spain of 105 individuals with diabetic retinopathy found a slowing of progression in the group taking antioxidant supplements compared with the control group but saw no changes in vision. The authors concluded that antioxidant supplementation could be a useful long-term adjunct therapy.12 Research is also mixed on whether dietary antioxidant intake reduces risk of glaucoma.13,14

Omega-3s
Despite their anti-inflammatory benefits, it's unclear whether omega-3 fatty acids have a positive, negative, or neutral effect on glaucoma risk, and there's no evidence that they prevent the onset or progression of cataracts or diabetic retinopathy.14

Omega-3 intake is inversely related to AMD risk.15-17 A 2015 Cochrane review articulated that evidence from population studies suggests that people who have a diet with relatively high levels of omega-3 fatty acids (such as those derived from fish oils) are less likely to develop AMD, but that supplementation of omega-3s for up to five years doesn't appear to slow AMD's progression.18

What RDs Should Know About Eye Disease

AMD
AMD affects the macula, the central area of the retina. The retina is the light-sensitive tissue lining the back of the eye that records the images we see and sends them to the brain. The macula is responsible for focusing central vision in the eye, so AMD impairs the ability to recognize faces or colors and see objects in fine detail.

Because AMD treatment is both costly and often ineffective, there's considerable interest in the role of modifiable risk factors, including diet, to prevent AMD or slow its progression.15 Lifestyle modification can reduce the risk of early AMD, which would also reduce the lifetime risk of developing advanced AMD.19

AMD is more common in individuals with a history of obesity or lifestyle-related chronic diseases, including cardiovascular disease, type 2 diabetes, hypertension, and other diseases related to inflammation. Intake of higher-glycemic foods is consistently associated with increased risk of AMD.15-17 Thus, dietitians can play an important role in educating patients with these chronic diseases about the importance of regular eye exams and the additional benefits of improved nutrition. Blazier says that, in addition to the AREDS formula for macular degeneration, optometrists often recommend a well-balanced diet with plenty of leafy green vegetables to provide lutein.

Cataracts
Cataracts are the leading cause of correctable vision impairment. A cataract occurs when the normally clear lens in the eye becomes cloudy. In most cases, cataracts develop slowly as a part of normal aging, but adults with diabetes are two to five times more likely to develop cataracts than people without diabetes, according to the National Eye Institute.20

This is important information that dietitians can communicate to their patients with diabetes, along with the benefits of antioxidant- and carotenoid-rich fruits and vegetables.

Diabetic Retinopathy
Diabetic retinopathy affects the delicate blood vessels in the retina, causing them to leak or bleed, distorting vision.21 It's the most common microvascular complication—and the primary cause of vision loss—in people with diabetes and the leading cause of impaired vision and blindness among working-age adults, according to the NIH.1

Nutritional intervention for diabetes is essential to help patients maintain good blood sugar control in order to prevent the chronic hyperglycemia that causes blood vessel damage and leads to not just diabetic retinopathy but other complications of uncontrolled diabetes as well.

"When dealing with the diabetes population in the exam chair, we emphasize the importance of tight blood sugar control and avoiding the roller coaster of blood sugar, which is more likely to cause bleeding in the retina, progression of cataracts, and permanent vision loss," Blazier says.

Glaucoma
Glaucoma is a group of diseases that damages the eye's optic nerve, which can result in vision loss or blindness. While elevated intraocular pressure (pressure within the eye) is a major risk factor, secondary glaucoma is a complication of poorly controlled diabetes or hypertension. Having diabetes doubles the risk of developing glaucoma, according to the National Eye Institute,22 but analysis of National Health and Nutrition Examination Survey (NHANES) data has questioned this association.23

While nutrition can't reduce eye pressure, it plays a fundamental role in preventing or managing both diabetes and hypertension. Damage to the optic nerve can occur even in the absence of intraocular pressure, likely due to oxidative stress and free radical damage to nerve cells.1

Caffeine temporarily increases intraocular pressure, but moderate intake hasn't been shown to increase glaucoma risk. However, heavy caffeine use may increase the risk of some forms of glaucoma, and individuals with a family history of primary open-angle glaucoma related to eye pressure may be at particular risk.13,24

NHANES data from 2007–2008 suggest that excessive calcium and iron intake increases the risk of glaucoma.25 Dietary intake appears to be protective, but supplemental intake doesn't.26 Intake of 800 mg/day or more of supplemental calcium or 18 mg/day or more of supplemental iron was associated with higher presence of glaucoma compared with no supplemental intake. Intake of both supplements further increased risk.25 Patients with established deficiencies should continue to supplement, but this may be a case where more isn't better.

Great Opportunity
Dietitians are well versed in the many reasons patients benefit from filling one-half of their plate with fruits and vegetables. With the role of carotenoids and antioxidants in preventing the onset or progress of vision-stealing eye diseases, RDs have a likely underused avenue for tapping into patients' motivation to make dietary changes.

— Carrie Dennett, MPH, RDN, CD, is the nutrition columnist for The Seattle Times and speaks frequently on nutrition-related topics. She also provides nutrition counseling via the Menu for Change program in Seattle.


References

1. Pinazo-Durán MD, Gallego-Pinazo R, García-Medina JJ, et al. Oxidative stress and its downstream signaling in aging eyes. Clin Interv Aging. 2014;9:637-652.

2. Yang SF, Roberts JE, Liu QH, Pang J, Sarna T. Zeaxanthin and lutein in the management of eye diseases. J Ophthalmol. 2016;2016:4915916.

3. Wu J, Cho E, Willett WC, Sastry SM, Schaumberg DA. Intakes of lutein, zeaxanthin, and other carotenoids and age-related macular degeneration during 2 decades of prospective follow-up. JAMA Ophthalmol. 2015;133(12):1415-1424.

4. Chew EY, Clemons TE, SanGiovanni JP, et al. Lutein + zeaxanthin and omega-3 fatty acids for age-related macular degeneration: the Age-Related Eye Disease Study 2 (AREDS2) randomized clinical trial. JAMA. 2013;309(19):2005-2015.

5. Chew EY, SanGiovanni JP, Ferris FL, et al. Lutein/zeaxanthin for the treatment of age-related cataract: AREDS2 randomized trial report no. 4. JAMA Ophthalmol. 2013;131(7):843-850.

6. Chew EY, Clemons TE, SanGiovanni JP, et al. Secondary analyses of the effects of lutein/zeaxanthin on age-related macular degeneration progression: AREDS2 report no. 3. JAMA Ophthalmol. 2014;132(2):142-149.

7. Sahli MW, Mares JA, Meyers KJ, et al. Dietary intake of lutein and diabetic retinopathy in the Atherosclerosis Risk in Communities Study (ARIC). Ophthalmic Epidemiol. 2016;23(2):99-108.

8. Evans JR, Lawrenson JG. Antioxidant vitamin and mineral supplements for preventing age-related macular degeneration. Cochrane Database Syst Rev. 2012;(6):CD000253.

9. Evans JR, Lawrenson JG. Antioxidant vitamin and mineral supplements for slowing the progression of age-related macular degeneration. Cochrane Database Syst Rev. 2012;11:CD000254.

10. Mathew MC, Ervin AM, Tao J, Davis RM. Antioxidant vitamin supplementation for preventing and slowing the progression of age-related cataract. Cochrane Database Syst Rev. 2012;(6):CD004567.

11. Yonova-Doing E, Forkin ZA, Hysi PG, et al. Genetic and dietary factors influencing the progression of nuclear cataract. Ophthalmology. 2016;123(6):1237-1244.

12. Garcia-Medina JJ, Pinazo-Duran MD, Garcia-Medina M, Zanon-Moreno V, Pons-Vazquez S. A 5-year follow-up of antioxidant supplementation in type 2 diabetic retinopathy. Eur J Ophthalmol. 2011;21(5):637-643.

13. Bussel II, Aref AA. Dietary factors and the risk of glaucoma: a review. Ther Adv Chronic Dis. 2014;5(4):188-194.

14. Pasquale LR, Kang JH. Lifestyle, nutrition, and glaucoma. J Glaucoma. 2009;18(6):423-428.

15. Schleicher M, Weikel K, Garber C, Taylor A. Diminishing risk for age-related macular degeneration with nutrition: a current view. Nutrients. 2013;5(7):2405-2456.

16. Sin HP, Liu DT, Lam DS. Lifestyle modification, nutritional and vitamins supplements for age-related macular degeneration. Acta Ophthalmol. 2013;91(1):6-11.

17. Weikel KA, Chiu CJ, Taylor A. Nutritional modulation of age-related macular degeneration. Mol Aspects Med. 2012;33(4):318-375.

18. Lawrenson JG, Evans JR. Omega 3 fatty acids for preventing or slowing the progression of age-related macular degeneration. Cochrane Database Syst Rev. 2015;4:CD010015.

19. Mares JA, Voland RP, Sondel SA, et al. Healthy lifestyles related to subsequent prevalence of age-related macular degeneration. Arch Ophthalmol. 2011;129(4):470-480.

20. Facts about cataract. National Eye Institute website. https://nei.nih.gov/health/cataract/cataract_facts. Updated September 2015. Accessed June 18, 2016.

21. Facts about diabetic eye disease. National Eye Institute website. https://nei.nih.gov/health/diabetic/retinopathy. Updated September 2015. Accessed June 19, 2016.

22. Facts about glaucoma. National Eye Institute website. https://nei.nih.gov/health/glaucoma/glaucoma_facts. Accessed June 19, 2016.

23. Ko F, Boland MV, Gupta P, et al. Diabetes, triglyceride levels, and other risk factors for glaucoma in the National Health and Nutrition Examination Survey 2005–2008. Invest Ophthalmol Vis Sci. 2016;57(4):2152-2157.

24. Pasquale LR, Wiggs JL, Willett WC, Kang JH. The relationship between caffeine and coffee consumption and exfoliation glaucoma or glaucoma suspect: a prospective study in two cohorts. Invest Ophthalmol Vis Sci. 2012;53(10):6427-6433.

25. Wang SY, Singh K, Lin SC. Glaucoma prevalence and the intake of iron and calcium in a population-based study. Curr Eye Res. 2013;38(10):1049-1056.

26. Wang SY, Singh K, Lin SC. The association between glaucoma prevalence and supplementation with the oxidants calcium and iron. Invest Ophthalmol Vis Sci. 2012;53(2):725-731.

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